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PARITY CHECKING
A vertical redundancy check (VRC) is performed on all
characters received by the 2845. Conversely, a VRC bit
is added to each character by the 2845 to establish even
parity before transmission to the I/O channel.
If the 2845 detects a VRC error in a control character
received from the I/O channel, the 2845 will not respond.
Failure to respond to the I/O channel will result in a time-
out at the 2701.
The action taken by the 2845 to a VRC error detected
in a text character received from the I/O channel depends
upon the device to which the text is addressed. If the
device is a Printer, a VRC error resets the 2845 and inhib-
its printing. The 2845 responds with NAK. If the device is
a Display Station, the Check character is displayed on the
screen in lieu of the character received in error, and the
2845 responds with NAK. Conversely, if the 2845 detects
an internal parity error during transmission of a message
to the I/O channel, the 2845 forces a CAN control charac-
ter prior to the ETX to notify the CPU that the transmitted
text contains characters with 2845 corrected parity. Si-
multaneously, a Check symbol, or symbols, is displayed on
the 2265 screen to give visual indication to the operator.
(See "Check Symbol".) At the CPU the characters with
corrected parity are identified as those immediately pre-
ceding the Check characters.
Finally, during all print operations (2845 printer-buffer-
to-l053 data transfers), each character is checked for an
internal parity error. If an error is detected during any
print operation, the Check symbol is printed at the end of
the message. (See "Operation with 1053 Printer".)
An LRC (longitudinal redundancy check) character is
accumulated by the transmitting device (2701 or 2845)
during any transmission sequence that begins with STX and
ends with ETX. As the receiving device (2701 or 2845) re-
ceives each character, it also accumulates an LRC charac-
ter. Following the transmission of ETX, the transmitting
device sends its accumulated LRC character. When received
by the receiving device, this LRC character is compared to
the LRC character accumulated by the receiving device.
An equal comparison indicates successful transmission of
total number of bits. An unequal comparison indicates an
unsuccessful transmission; the 2845 responds with NAK.
The LRC and the STX characters are not· included in the
LRC accumulation of either device.
PROGRAMMING NOTE: LRC generation and accumulation are
performed internally in the 2701. Therefore, the program need
not include an LRC character in any sequence of characters to,
nor wilt it receive an LRC character from, the I/O channel.
ADDRESSING SEQUENCE
The 2265/2845 display system is designed to operate on
communications networks wherein the central computer
28
operates as the control station. In this centralized system,
the computer initiates all contacts with the Display Control
units attached to the network. The computer may poll a
number of Display Controls (terminals) and their attached
components (Display Station and Printer) in turn until one
is found that has a message ready to send to the computer.
Or, the terminal may address a particular Display Control
when it has a message to send to one of the components
attached to the Display Control. In either case, the com-
puter must initiate the contact; the 2845 Display Control
does not have the capability to initiate the contact. In
summary, message flow from the terminal to the computer
is achieved by polling the terminal; message flow from the
computer to a terminal is achieved by addressing the ter-
minal. Since the format of the four-character sequence
required to initiate communication is the same in all cases,
this sequence is referred to as the "addressing sequence"
in this manual.
Each 2845 command issued by the I/O channel to the
remote display system is transmitted to the 2845 Display
Control as one of the four bytes in this addressing se-
quence. The purpose of the addressing sequence is to place
all 2845's on the multidrop network in control mode, select
the desired 2845 and attached 2265 or 1053, and specify
(command) the operation to be performed. The address-
ing sequence of each command is shown in Figures 6
through 13.
The specific function of each of the four bytes of the
addressing sequences is described below:
First Byte - The first byte of an addressing sequence
must be either an SOH or EOT communication control
character. The SOH or EOT communication control char-
acter places the 2845 Display Control in control mode.
Second Byte - The second byte in the addressing se-
quence contains the address of the 2845. Table 3 lists the
permissible 2845 Display Control addresses.
Third Byte - The third byte contains the address of
the remote device (2265 Display Station or 1053 Printer)
to be selected. Table 4 lists the address assignments for the
2265 Display Station and 1053 Printer.
Fourth Byte - The fourth byte of an addressing se-
quence specifies the command to be executed by the
I
selected 2845 Display Control. Table 5 lists the 2265/2845
commands.
Each of the fou;-byt/s' of every addressing sequence is
parity-checked (VRC only) by the 2845 as it is received.
If a parity error is detected or if any byte of the addressing
sequence
is
not vaii<lol' is oulofsequeace, the 2845 will
not respond.
.
..
To maintain operational continuity, the 2845 must
respond to an addressing sequence within 2 seconds after
the addressing sequence is transmitted to the 2845. Failure
to respond (no response) results in a 2701 line timeout.
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